The present invention relates to a method of repairing turbine blades.
Turbine blades on rotors and stators are subjected to wear and damage during use and after a certain period of use servicing of the rotors and/or stators is necessary, such servicing often requiring welding operations to be carried out to the blades on the rotor to repair damaged or eroded parts.
It has been proposed by the applicants that repairs to turbine blades wherever possible are carried out with the blades in situ on the rotor thereby obviating their removal, which operation is not only time consuming but can in itself cause considerable damage to the rotors or blades and necessitates the removal of cover bands, shrouds, lacing wires etc.
U.S. patent application Ser. No. 07/53996, filed Jun. 18, 1990, discloses apparatus to support a rotor. which apparatus also includes a work station to enable repair operations to be carried out on turbine blades in an array of blades on a rotor without their removal from the rotor.
Whereas such apparatus has been found to be highly successful and enables swift and accurate repairs to be carried out to the blades, quite considerable time can be spent in setting up the machining apparatus, drills, cutting apparatus etc.
The main reason for the considerable expenditure of time is that repair on the blades which normally necessitates removal of lacing wires, cover bands or other blade tying or damping means, results in the blades being considerably misaligned from a normal in-use position and not only misaligned but misaligned relative to each other.
There can be various reasons for such misalignment, if lacing wires are not continuous there is a tendency for the end blades in a package of blades, the end blade only being effectively tied to a blade on one side rather than both sides, for that blade to move or unwind during use of the turbine and removal of the lacing wires accentuates such unwinding.
Furthermore, in some cases the blades may have been poorly manufactured or installed on the rotor and, since lacing wire holes would have been pre-drilled, the blades would have been physically forced into a position so that the lacing wires can be threaded through the blade and thus on their removal the blade will immediately take up a misaligned position.
It will be appreciated that a very small error in securing the blade at its root end can, particularly in the case of a blade of considerable length in excess of one meter, for example, result in the tip of the blade being considerably displaced from its proper position.
The tendency for previously tied blades released from their "tied condition" to take up positions considerably displaced from their normal in-use aligned position not only causes considerable problems when it is required to carry out intricate welding and machining on the blades to carry out any necessary repair, but since it is also necessary to eventually return the blades to their proper position, this may require the application of considerable physical force and possibly heat after the repair has been carried out and possibly after heat treatment processes have been carried out in order to restore the blades to the correct position.
Such may be the application of physical force and heat that further heat treatment processes may then be necessary further prolonging the time taken for the total repair of the turbine.
Furthermore, where machining such as the re-drilling of lacing wire holes is to be carried out, if the blades are not in an aligned position it can be very difficult to accurately drill the holes so that the lacing wires can be successfully re-threaded through the blades.